Briefly, these keep the backs of C and early 900 cars from "self adjusting", "popping"
Because the adjustment mechanism is so cleverly designed, it is not easy to explain how they do that, but suffice to say that when the plastic hinge cover on those cars breaks and falls off, there is nothing to keep the seat from deciding where IT wants the seat back.
These are fiddly little bits of stainless, machined on all surfaces, with very tight tolerances, which act as thrust surfaces to keep the seats working as they did when they left the factory.http://www.precisionmatters.biz/seat-po ... oppers.php

If yours has that pin (in the red ellipse), the Popper Stoppers won't help. It does what the Popper Stoppers do.
Some guy at the factory (who probably drove a C with broken plastic covers) figured out sometime in the mid-'67 MY run that there was a problem and became a hero at the engineering meeting by offering a 50-pfennig fix; spend 45-pfennigs to drill the hole and put that 5-pfennig pin in there.
If you could get yours disassembled enough to do so, nobody would buy my parts, but you can't without ending up re-chroming the parts.
Anyhow, it's now narrowed down to that time-frame, since a customer has a mid-'67 900 car with the pin in the driver's side mechanism and no pin on the passenger's side.
You didn't expect them to pitch the mechanisms without the pin, did you?